1. Field of the Invention
The present invention relates to an information storage medium and a method of manufacturing the same, wherein a portion of a recording layer is irradiated with a light beam such as a laser beam to reverse the direction of magnetization in the irradiated portion, thereby recording or erasing information.
2. Description of the Prior Art
A magneto-optical information storage medium containing a rare earth-transition metal amorphous alloy utilizing a polar Kerr effect is a known information storage medium which is irradiated by a light beam, such as a laser beam, in order for information to be recorded thereon, reproduced, or erased therefrom. The recording layer of this storage medium consists of a magnetic alloy, with the axis of easy magnetization being perpendicular to the surface of the recording layer.
Upon radiation of a light beam modulated in accordance with an information signal, the recording layer of such an information storage medium is locally heated to a temperature closer to the Curie point, with the result that the coercive force of the irradiated portion becomes lower than that of an external magnetic field. Consequently, the direction of magnetization of the irradiated portion is reversed by the external magnetic field, thereby recording information thermomagnetically.
In order to reproduce the recorded information, the linear polarized light beam is emitted on the recording layer. Rotation of the plane of polarization of light reflected by the surface of the recording layer, that is, the polar Kerr effect (i.e., a thermo-optical effect), is utilized to reproduce the recorded information.
In a magneto-optical information storage medium utilizing the polar Kerr effect, a thin film of a rare earth-transition metal amorphous alloy such as TbFe, GdTbFe, TbFeCo, or GdTbFeCo is used. These amorphous alloy thin films have relatively low Curie points and good film formation properties to provide a large and uniform film. In addition, these films have high recording efficiency and require only a small amount of optical energy for recording. The amorphous alloy thin films have excellent properties as recording layers for optomagnetic information storage media.
If an information storage priority area and a rewritable area can be provided in a recording layer of an information storage medium of the type described above, information can be selectively stored in the medium in accordance with types of information, thus resulting in convenience. A strong demand has arisen for such an information storage medium. Since a recording layer of a magneto-optical information storage medium is formed by sputtering or vacuum evaporation, it can be formed on the entire area of a substrate with uniform magnetic properties. It is, therefore, difficult to form the two areas described above in one recording layer.
One drawback of a rare each-transition metal amorphous alloy is tends to become oxidized when it is exposed to a high-humidity environment. Therefore, if this type of alloy is used to form a recording layer and is left unprotected, the recording layer may well become oxidized. If this occurs, polar Kerr rotation angle .theta..sub.K, which is one of the magneto-optical properties of the recording layer, will then decrease significantly, and the relationship between the angle .theta..sub.K and the coercive force of the recording layer cannot be represented by a rectangular polar Kerr hysteresis loop. As a result, the recording characteristic of the recording layer will be degraded.
Therefore, to prevent oxidation of the recording layer from occurring, a protective layer must be formed thereon. However, it has been found that if the protective layer is made of an oxide such as SiO or SiO.sub.2, deposited by vacuum deposition without a heat treatment, as has been in the conventional information storage media, free oxygen atoms will be released therefrom, inevitably oxidizing the recording layer. This is pointed out in T. Deguchi et al., Appl. Optics, 23 3972 (1984).